The mammalian Na+/H+ antiporters NHE-1, NHE-2, and NHE-3 are electroneutral and voltage independent, but can couple to an H+ conductance.

Abstract

Na+/H + exchange in vertebrates is thought to be electroneutral and insensitive to the membrane voltage. This basic concept has been challenged by recent reports of antiport-associated currents in the turtle colon epithelium Dawson, 1992, 1994). To determine the electrogenicity of mammalian antiporters, we used the whole-cell patch clamp technique combined with microfluorimetric measurements of intracellular pH (pHi). In murine macrophages, which were found by RT-PCR to express the NHE-1 isoform of the antiporter, reverse (intracellular Na+-driven) Na+/H § exchange caused a cytosolic acidification and activated an outward current, whereas forward (extraceUular Na+-driven) exchange produced a cytosolic alkalinization and reduced a basal outward current. The currents mirrored the changes in pHi, were strictly dependent on the presence of a Na § gradient and were reversibly blocked by amiloride. However, the currents were seemingly not carried by the Na+/H + exchanger itself, but were instead due to a shift in the voltage dependence of a preexisting H § conductance." This was supported by measurements of the reversal potential (Erev) of tail cur-" rents, which identified H + (equivalents) as the charge carrier. During Na+/H + exchange, Er,v changed along with the measured changes in pHi (by 60-69 mV/ pH). Moreover, the current and Na § + exchange could be dissociated. Zn 2 § which inhibits the H § conductance, reversibly blocked the currents without altering Na+/H + exchange. In Chinese hamster ovary (CHO) cells, which lack the H + conductance, Na+/H § exchange produced pHi changes that were not accompanied by transmembrane currents. Similar results were obtained in CHO cells transfected with either the NHE-1, NHE-2, or NHE-3 isoforms of the antiporter, indicating that exchange through these isoforms is electroneutral. In all the isoforms tested, the amplitude and time-course of the antiport-induced pHi changes were independent of the holding voltage. We conclude that mammalian NHE-1, NHE-2, and NHE-3 are electroneutral and voltage independent. In cells endowed with a pH-sensitive H + conductance, such as macrophages, activation of Na+-H §

Sodium Influx Pathways during and after Anoxia in Rat Hippocampal Neurons

“…7C). We also reported previously that a Zn 2ϩ -sensitive H ϩ efflux pathway contributes to acid extrusion after anoxia in rat hippocampal neurons and that inhibition of this pathway increases the observable contribution of Na ϩ /H ϩ exchange to acid extrusion at this time (Diarra et al, 1999;Sheldon and Church, 2002) (see also Demaurex et al, 1995 (Fig. 7 B, C), lending additional support to the possibility that Na ϩ /H ϩ exchange contributes to Na ϩ influx immediately after anoxia.…”

A pH-stabilizing role of voltage-gated proton channels in IgE-mediated activation of human basophils

“…Importantly, Zn 2ϩ unmasked occult proton efflux during the response, thereby indicating that proton channels are most likely responsible. Other plasma membrane proton extrusion mechanisms (Na ϩ /H ϩ -antiport and H ϩ -ATPase) are insensitive to Zn 2ϩ (35,36). Together, these results indicate that voltage-gated proton channels in basophils may help keep pH i in an optimal range for histamine secretion.…”

Phylogeny and cloning of ion transporters in mosquitoes

“…Mammalian NHEs mediate the exchange of one proton for each cation translocated and thus are electroneutral and membrane potential insensitive (Aronson, 1989;Demaurex et al, 1995;Post and Dawson, 1994). Interestingly, prokaryotes exhibit an electrogenic transport of 1Na + /2H + (Padan and Schuldiner, 1994), whereas crustacean transporters operate with a stoichiometry of 2Na + /1H + (Ahearn et al, 1994).…”